Dyes from textile industries are one of the majors water pollutants source. These substances reduce the uptake of light, interfering with aquatic plants photosynthesis hence having a direct impact in the oxygen content of water sources, progressive eutrophication and bioaccumulation of these substances (Al-Nakib. C., et al. 2009). Everyday textile industries dispose large quantities of dye-contaminated waters into the rivers (Sen S., et al, 2003). It is estimated that among between 40 to 50% of the initial dye concentration ends up in wastewaters (Bhatnagar, A., et al. 2005). Therefore, it is necessary to innovate in order to obtain new alternatives for dyes contaminated waters treatment.
There are several technologies to treat wastewaters contaminated with dyes. Being relevant the H2O2 oxidation activated with UV light (Muruganandham, M., et al. 2004), advanced oxidative processes using Fenton reactant (Arslan, D., et al. 2000), electrochemical flocculation (Yang, C., et al. 2005), sorption by cucurbiturils (cyclic polymer formed from glycoluryl and formaldehyde) (Robinson, T., et al. 2001) and biological treatments (using either bacteria or fungi) (An, H., et al. 1996). These technologies used to remove wastewaters color are expensive and can produce secondary pollutants, by transferring those contaminants from liquid to solid phase (Burtscher, E., et al. 2004). Therefore, it is necessary to contribute developing new, non-expensive and secondary pollutant-free technologies for removal of color from wastewaters for short or long term.
Recently, nanostructured manganese oxides have drawn attention because of promising applications that these materials could have in several industrial processes. For instance, as main components in battery systems and as catalyst for a wide range of industrial processes. MnO2 is the most important of manganese oxides, having more than 14 polymorphs and being widely used given that its internal structure has cavities or pockets with similar sizes to alkaline and alkaline earth cations, and heavy metals cations Therefore, MnO2 is a promissory material for potential applications as molecular sieve and water softening systems (Stobbe, E., et al. 1999).
There are several abstracts and disclosures describing MnO2 nanoparticles synthesis and characterization in solution. The most common procedure involves a reduction of Mn(VII) salts with a suitable reducing agent (Stobhe, E., et al. 1999). Sonochemical aquous reduction of Mn(VII) in presence of surfactants have also been used to synthesize MnO2 nanoparticles in solution. Disclosures show that nanoparticles of MnO2 obtained using ultrasound are more homogeneous in size and shape. In addition, it has been reported that a longer exposure to ultrasound leads the reduction from Mn(IV) to Mn2+(Stobbe, E., et al. 1999).
The interesting properties observed in nanoparticles in solution indicate that these materials may have more versatility in applications if synthesized in solid phase immobilized on a holder. Thus the processes of nanoparticles “protection” with monolayers of organic molecules that might prevent the use of the material surface are avoided. Synthesis methods contemplating, one step synthesis of manganese dioxide nanocrystals inside the channels of mesoporous silica (SBA-15) using manganese nitrate as precursor. MnO2 nanocrystals forming in SBA-15 was performed by microwave digestion, achieving that 40% of the mesoporous support volume to be occupied with the metal oxide (Zhu, S., et al. 2006).
In 2004, it was developed an effective technique named “two solvents method” which allowed complete filling of the pores of SBA-15. The first step of the synthesis was to incorporate inorganic precursors into the SBA-15 channels through two kinds of solvents, which facilitate the transfer of the precursor ion from the reaction mixture to the surface of the mesoporous silica channels. The next step was to connect the precursors into the pores by means of thermal treatment, which induced transformation into oxide nanoparticles. The disadvantage of this method was presented by the inability to control the synthesized oxide morphology (Zhu, S., et al. 2006).
In parallel, sonochemical methods have been used to generate novel materials in short reaction times. It has been reported that nanocrystalline materials of oxides obtained by sonochemical methodologies exhibit high purity compared to the materials obtained by conventional methods (Zhu, S., et al. 2006). The chemical effect of ultrasound is due to acoustic cavitation phenomenon, which causes the formation, growth and collapse of bubbles in a liquid, giving particular properties to solutions irradiated with ultrasound waves. Acoustic cavitation produces water and other aqueous solutions reactive radicals. These reactive radicals which have temperature, pressure and extremely high cooling rate, can reduce metal ions to metal or metal oxides nanoparticles (Zhu, S., et al. 2006).
In 2006, Shenmin Zhu et al (Zhu, S., et al. 2006), reported the synthesis of mesoporous MnO2 using SBA-15 as a silica matrix and KMnO4 in an aqueous HCl solution as precursor, in the presence of ultrasonic waves for 6 hours. The analysis of X-ray diffraction revealed that the MnO2 reaction product was amorphous with a small amount of nanocrystalline phase. Additionally, electron microscopy analysis confirmed the homogenous synthesis of nanostructures.
As shown, the mesoporous silica and the surfactant substances have been widely used in the preparation of nanostructured oxides. In recent years there has been increased interest in using biological supports because of their interesting structures, which can be useful in the synthesis of nanoparticles. Compared to other media, biological materials are “ecofriendly” and easy to obtain. The structure of biological materials provides stable and controllable conditions in the assembly of nanostructured oxides, which can replicate the template morphology and even its functionalities (Wang H., et al. 2010).
In 2009, Huan-qin Wang and colleagues (Wang, H., et al., 2010) reported the synthesis of MnO2 microfibers with a secondary nanostructure using cotton as support and KMnO4 as precursor. Cotton was treated with aqueous solutions of HCl and NaOH in order to improve its porous structure and create the appropriate ionic environment for MnO4− ions insertion in the fiber. A small amount of cotton treated was dispersed in an aqueous solution of KMnO4, and irradiated subsequently with ultrasonic waves for 6 hours. Finally, the sample was dried and calcined at 500° C. By the treatment described above, MnO2 microfibers were obtained, on which MnO2 nanorods and nanoparticles grew up whereas KMnO4 concentration increased during the synthesis.
Manganese oxides are very strong oxidative agents. It have been reported that oxides and hydroxides of Mn3+ and Mn4+ can oxidize inorganic contaminants such as Cr(III) and Co(II) complexes as well as organic contaminants like substituted phenols, aromatic amines, explosives and pesticides. In 2009, Al-Nakib. C. Chowdhury et al. (Al-Nakib. C., et al. 2009) tested the catalytic activity of Mn3O4 NPs towards the oxidative degradation of dyes in water.
In 2010, Cao et al Guangsheng (Cao, G., et al. 2010) reported a hydrothermal method of MnO2 α-β nanorods synthesis evaluating its catalytic efficiency in the oxidation of Rhodamine B (RB) and MB. Catalytic oxidation of the dyes was carried out by mixing H2O2 solutions with a certain amount of MnO2 nanorods (Phases MnO2-α and MnO2-β separately). UV-Vis Spectra of the reaction mixtures showed that H2O2 does not discolor solutions by itself. The use of a catalyst alone allowed a discoloration degree of 8% in 20 minutes. The combination of the catalyst and H2O2 allowed a 95% RB solution bleaching in 90 minutes. In the case of the MB solution, greater discoloring was achieved when MnO2-β nanorods were used compared when MnO2-α nanorods were used. The calculation of the surface areas of the catalysts revealed that the MnO2-β phase has a greater surface area and therefore more exposed catalyst sites compared to the MnO2-α phase.